Untitled DocumentA time zone is a region on Earth that has a uniform standard
time for legal, commercial, and social purposes. In order for the same clock time
to always correspond to the same portion of the day as the Earth rotates (for
example, the sun being at its highest point every day around noon), different
places on the Earth need to have different clock times. Time zones have been used
in modern times so similarly situated cities can keep exactly the same time, for
simplicity and ease of communication.

Los Angeles

New York

London

Moscow

Tokyo

Standard time zones could be defined by geometrically subdividing the Earth's
spheroid into 24 lunes (wedge-shaped sections), bordered by meridians each 15Â°
of longitude apart. The local time in neighboring zones would differ by one
hour, and the variation in the position of the sun from one end of the zone
to the other (east vs. west) would be at most 1/24th of the sky. Most of the
25 nautical time zones (specifically UTCâˆ’11 to UTC+11) are indeed defined this
way, and are 15Â° of longitude wide. An hourly zone in the central Pacific Ocean
is split into two 7.5Â° wide zones (UTCÂ±12) by the 180th meridian, part of which
coincides with the International Date Line.

On land, it is more convenient for areas in close commercial or other communication
to keep the same time, so time zones tend to follow the boundaries of countries
and their subdivisions instead. Of the 40 time zones on land, most are offset
from Coordinated Universal Time (UTC) by a whole number of hours (UTCâˆ’12 to
UTC+14), but a few are offset by 30 or 45 minutes from a nearby hourly zone.
Daylight saving time is used in some higher-latitude countries to manipulate
clock time with respect to the position of the sun for parts of the year, typically
by changing clocks by an hour. Many land time zones are skewed toward the west
relative to the corresponding nautical time zones, which also creates a permanent
daylight saving time-like offset. Computer operating systems use either UTC
or a local time zone to time stamp events.